1. Field of the Invention
The present invention relates to a die bonding apparatus for bonding a semiconductor pellet to a semiconductor device package such as a stem or a lead frame by using a hard solder such as AuSn, AuGe and AuSb as a bonding agent.
2. Description of Related Art
In the prior art, in case of a semiconductor device such as a microwave semiconductor device having electric characteristics susceptible to influence of heat, a hard solder such as AuSn, AuGe and AuSb having a good heat conductivity has been used for bonding a semiconductor pellet. However, when die bonding is carried out by using this kind of bonding agent, an oxide film can be formed on a surface of a molten bonding agent by oxidation in the open air, so that a void is generated a: a boundary between this oxide film and the semiconductor pellet, with the result that a sufficient bonding strength cannot be obtained.
Ordinarily, the bonding is carried out in an inert gas atmosphere covered with an atmosphere cover, so as to prevent oxidation. Furthermore, even if a thin oxide film is generated, the semiconductor pellet is pressed down by a pincette so that a scribing operation is used to remove the thin oxide film before the bonding. However, even if the scribing operation is carried out in the inert gas atmosphere, oxygen is present in the open air, so that oxidation occurs. Because of this, the formed oxide film cannot be completely removed with only the scribing operation, with the result that voids occur in the oxide film remaining at the boundary between the bonding agent and the semiconductor pellet or the stem. As a result, there was a problem that heat resistance becomes large, so that a desired characteristic cannot be obtained or the semiconductor pellet is destroyed because of an elevated temperature.
FIG. 5 is a front view illustrating one example of the prior art die bonding apparatus, which is disclosed in Japanese Patent Application Pre-examination Publication No. JP-A-06-029331 as a die bonding apparatus in order to overcome the above mentioned problem. As shown in FIG. 5, this die bonding apparatus includes a work placing table 29 having a heating mechanism for placing thereon a semiconductor device package 33 which is the stem, a solder supplying mechanism 32 for cutting a bonding agent 34 into an arbitrary size and for supplying and locating the arbitrary size of bonding agent on an upper surface of the semiconductor device package 33, and a die bonding head 28 for locating a semiconductor device 35 on the bonding agent 34 placed on the semiconductor device package 33 so as to carry out a die bonding.
A periphery of the work placing table 29 is surrounded by an atmosphere cover 30 of a triple cover structure, and an inert gas is supplied between each pair of adjacent atmosphere covers 30, so that gas layers having different pressures are generated. Each of inert gas flow rate setters 31 sets the flow rate of the inert gas which is supplied through a pipe to between each pair of adjacent atmosphere covers 30, so that pressure differences are generated among the inert gas layers.
Now, an operation of this die bonding apparatus will be described. First, the bonding agent 34 is supplied on the upper surface of the stem of the semiconductor device package 33 which is heated on the work placing table 29, by means of the solder supplying mechanism 32. The bonding agent 34 is molten by the heating of the work placing table 29. Then, the semiconductor device 35 is placed on the molten bonding agent 34 by means of the die bonding head 28.
At this time, when a first layer of inert gas is injected from an opening of an innermost atmosphere cover 30, the first layer of inert gas is prevented from catching the air by a second layer of inert gas. Furthermore, when a nozzle of the solder supplying mechanism 32 and a nozzle of the die bonding head 28 are inserted into the opening, these nozzles are prevented from destroying the inert atmosphere by having air therein, by action of a third layer of inert gas which prevents intrusion of air.
As mentioned above, since the three inert gas layers having different gas pressures are formed at the opening of the atmosphere cover 30, the open air is excluded from being in the inside of the atmosphere cover, so that generation of the oxide film is prevented when the bonding agent 34 is in a molten condition.
In the above mentioned prior art die bonding apparatus, for the scribing operation of the semiconductor pellet by the pincette and for positioning of the stem on the work placing table, the opening of the fixed atmosphere cover has to be larger than the size of the stem by a degree large enough to enable to insertion not only the stem but also the pincette through the opening. However, if the opening, which is determined by the size of the stem constituting the semiconductor device package, becomes too large, it becomes impossible to prevent the entry of air even if the conditions of the pressures and the flow rates of the three layers of inert gas are changed. As a result, there occurs a problem that the oxidation of the bonding agent cannot be avoided and therefore the voids are generated.
Furthermore, in connection with the above mentioned problem, this type of semiconductor device package includes various sizes from a large size to a small size. Therefore, the above mentioned prior art apparatus is required to previously prepare a number of atmosphere covers having different sizes of opening which meet with different sizes of semiconductor device packages, respectively. This means that when the kind of the product is changed, the atmosphere cover must be exchanged and adjusted. Therefore, a considerable time is expended, so that productivity becomes low.
In addition, since this die bonding apparatus was mainly adapted to a small size of semiconductor package, the semiconductor device package could be heated to a predetermined temperature for a relatively short time by the heat from the work placing table, only by placing the semiconductor device package on the work placing table. However, if the stem which is the semiconductor device package becomes large, not only the volume becomes large, but also a gap occurs at a contacting plane between the semiconductor device package and the work placing table, so that a sufficient heat conduction cannot be obtained, with the result that there occurs a problem that a considerable heating time becomes necessary. In addition, necessity of the considerable heating time results in acceleration of the oxidation of the bonding agent, so that the above mentioned problem of voids occur more and more.